Steel parts and method for heat-treating steel parts

ABSTRACT

The invention relates to components made of steel, more particularly outer joint parts and inner joint parts of constant velocity joints, and to a process of heat treating such components made of steel. The heat treatment operation includes the process stages of nitriding, induction surface layer hardening and tempering, which processes follow one another. As a result of the nitriding operation, the joint parts are provided with a surface layer ( 15 ) including nitrides and a diffusion layer ( 18 ) positioned thereunderneath. The subsequent induction hardening process causes the diffusion layer ( 18 ) to be hardened, so that it comprises good supporting characteristics for supporting the surface layer ( 15 ) positioned above same.

The invention relates to components made of steel for establishingcontact with rolling contact members, more particularly outer jointparts (2) and inner joint parts (3) of constant velocity joints, as wellas sleeves and journals of ball-guided longitudinal plunging pieces, andto a process of heat-treating such components.

In constant velocity joints with an outer joint part and an inner jointpart and with torque transmitting rolling contact members which are heldin running grooves in the outer part and inner part and roll therein,there occur high pressures between the rolling contact members and therunning grooves in the inner part and outer part. Such loads which arepresent in the form of Hertzian pressure can lead to the formation ofpitting which, in the final analysis, causes the constant velocityjoints to fail. Similar conditions prevail in the case of ball-guidedlongitudinal plunging pieces provided for torque transmitting purposeswherein sleeves and journals each comprise longitudinal grooves in whichballs are held and in which the balls roll.

From DE 31 32 363 C2 there is known a process for partiallysurface-hardening running grooves in the outer joint part and innerjoint part of a constant velocity ball joint wherein the contact facesof the running grooves and of the cage, which contact faces guide theballs, are hardened by means of a formed inductor. The depth of hardnessof the running grooves amounts to 0.5 mm. The guiding faces of the innerjoint part and of the outer joint part, which guiding faces arepositioned between the running grooves and serve to guide the cage, arehardened by a second hardening process which produces smaller depths ofhardness. For this purpose there are provided laser or electron beamhardening processes or, alternatively, thermo-chemical surface hardeningprocess such as nitriding.

It is therefore the object of the present invention to providecomponents made of steel for establishing contact with rolling contactmembers as well as a process of heat-treating components made of steelwherein the running characteristics of the faces of the rolling contactmembers are improved, wherein wear protection is improved and thus theservice life of the components extended.

The objective is achieved by components of said type made of steelhaving a surface layer consisting of a pore containing seam of burntiron nitrides and, positioned thereunderneath, a non-metallic connectingzone of iron nitrides, having an adjoining diffusion zone ofnitrogen-enriched mixed crystals and precipitated nitrides in a matrixof a martensite-nitrogen structure, having an induction hardened layerat least in part superimposed on the diffusion zone, and having a coreof heat-treatable steel positioned thereunderneath.

Furthermore, the objective is achieved by a process of heat-treatingcomponents of said type made of steel, comprising the below-mentionedprocess stages which follow one another: nitriding, surface layerinduction hardening and tempering.

The advantage of the solutions in accordance with the invention is that,by applying a nitriding process preceding the induction hardeningprocess, there is produced an outer surface layer which consists of ironnitrides as well as an adjoining diffusion zone which consists ofnitrogen-enriched mixed crystals and precipitated nitrides in a matrixof N-martensite. The cooperation between the surface layer and thediffusion zone positioned thereunderneath is of the greatestsignificance. The diffusion zone which, initially, is relatively softrelative to the surface zone, after induction hardening, comprises agreater strength and hardness. Induction hardening after quenching,results in a hard martensite-nitrogen structure. In this way it isensured that the induction-hardened diffusion zone positioned underneaththe outer surface layer securely supports the surface layer. Theformation of pitting on the running faces of the rolling contact membersunder operational conditions is prevented, as a result of which theservice life of the components is prolonged.

According to a preferred embodiment it is proposed that the surfacelayer extends over the entire surface of the joint parts. Furthermore,at least partial regions of the components, more particularly therunning grooves and guiding faces, are induction-hardened.

According to a preferred embodiment, the surface layer has a thicknessranging between approximately 10 to 30 μm. The thickness of the surfacelayer, inter alia, depends on the nitriding process used. When usingplasma nitriding, it is possible to achieve surface layer thicknesses of10 to 20 μm, whereas in the case of salt bath nitriding, layerthicknesses of 20 to 30 μm are produced.

The surface layer comprises a pore containing seam of burnt ironnitrides and, thereunderneath, a connecting zone of iron nitrides. Thepore seam has a thickness of approximately 2 to 3 μm and, depending onthe nitriding process, the connecting zone complements the surface layerthickness to the above-mentioned thickness of 10 to 30 μm. The porecontaining seam of burnt iron nitrides comprises a porous structure. Inconsequence, lubricant can enter the pores during operation, as a resultof which lubrication between two contacting components can be improvedconsiderably. In this way, the fretting tendency of the two componentsand thus component wear are reduced. The thickness of the porecontaining seam can be reduced to nearly zero, if lubrication is lesscritical.

The connecting zone comprises ε-iron nitrides which comprise a highdegree of hardness and a high fatigue strength and a high wearresistance. Components with nitrided and subsequently induction-hardenedrunning faces thus comprise improved running characteristics, as aresult of which the service life is extended.

The diffusion zone is preferably approximately 0.8 mm thick. Thethickness of the induction-hardened layer amounts to approximately 1.8to 2 mm. As a result of the induction hardening operation which followsnitriding, the structure of the diffusion zone obtains a greaterhardness and strength. This is advantageous in that the relatively thinsurface layer is supported by the diffusion zone. In this way, the riskof pitting under operational conditions is reduced.

According to a preferred embodiment of the invention, the steel used forthe components is unalloyed. This is advantageous in that the materialcosts are relatively low. Preferably, use is made of Cf 53, i.e. steelwith a carbon content of 0.53%.

In an alternative embodiment of the invention, the base material of thecomponents is alloy steel which is more expense than unalloyed steel,but has higher strength values after heat treatment.

The presence of alloying elements is not excluded.

The nitriding processes used to put the invention into effect are plasmanitriding, salt bath nitriding or gas nitriding. Gas nitriding requireslong nitriding times of approximately 100 hours for a nitriding depth of0.6 mm. As a result of ionisation of the nitrogen through glow dischargeduring plasma nitriding and by applying additional measures such asadding hydrogen and methane, the nitriding times can be shorter than inthe case of gas nitriding. Even shorter nitriding times are achieved bysalt bath nitriding, but the cyanide salt baths used always lead to aslight recarburisation of the surface layer.

According to a preferred embodiment of the inventive process, thecomponents are nitrided at a temperature of approximately 580° C. Inprinciple, it is also possible to nitride at lower temperatures, but inthat case, inward diffusion of the nitrogen into the component takeslonger.

According to a preferred further embodiment of the process, surfacelayer induction hardening is carried out at a middle frequency, with thedepth of current penetration and thus also the effective depth ofhardening decreasing with an increasing frequency. According to anadvantageous embodiment, the middle frequency ranges between 10 and 20kilohertz. With a frequency of 10 kilohertz, the effective depth ofhardening amounts to approximately 1.8 to 2 mm and with a frequency of30 kilohertz, the effective depth of hardening ranges between 1.5 and1.7 mm.

Tempering preferably takes place at a temperature of approximately 175°C. For the purpose of reducing hardness stresses, the components areheated in a furnace to said temperature which is then held for a periodof approximately 1 hour. Higher tempering temperatures can lead to areduced hardness of the diffusion zone and induction-hardened layer.

According to a preferred embodiment of the process it is proposed that,prior to being nitrided, the components are normalised. Alternatively,the components can be heat-treated prior to being nitrided. To achieve ahigh degree of toughness and a fine grain at a certain tensile strengthvalue, the components, during the heat treatment process, are quenchedand tempered.

A preferred embodiment will be explained with reference to the drawingswherein

FIG. 1 is an axial section through an inventive constant velocityplunging joint.

FIG. 2 is a cross-section through the inventive constant velocityplunging joint according to FIG. 1.

FIG. 3 shows part of the surface layer after nitriding and inductionhardening.

FIGS. 1 and 2 will be described jointly below. They show a constantvelocity plunging joint 1 in the form of a VL joint with an annularouter joint part 2 and an annular inner joint part 3 as well as balls 4for transmitting torque between the two joint parts, and furthermore acage 5 for guiding the balls 4. The outer joint part 2 comprises acylindrical guiding face 6 for guiding the cage 5, and outer runninggrooves 7. The inner joint part 3 comprises an outer face 8 and an innerrunning grooves 9. The inner and outer running grooves 7, 9 formcircumferentially alternating angles of intersection with thelongitudinal axes of the outer joint part 2 and of the inner joint part3. In inner and outer running grooves 7, 9 associated with one anotherthere is guided a ball 4 held in a window 10 of the cage 5.

The outer joint part 2 is provided with bores 11 for bolting on aflange. The inner joint part 3 is provided with a longitudinally toothedaperture 12 for inserting a shaft. (not illustrated).

The outer joint part 2 and the inner joint part 3 consist of anunalloyed heat-treatable steel with a mean carbon content of 0.53% (Cf53). Prior to being heat-treated, the joint parts are present in thenormalised condition. In the course of heat treatment, the two jointparts 2, 3 are first nitrided, then induction-hardened and finallytempered. Prior to being heat-treated, the joint parts 2, 3 had alreadybeen precision-machined. There is no provision for further machining ofthe surfaces of the joint parts 2, 3 after heat treatment.

For nitriding the joint parts 2, 3, preference is given to plasmanitriding. In the case of plasma nitriding, nitriding takes place in anitrogen-containing plasma which is produced with the help of glowdischarging in a vacuum furnace. By applying a high direct currentbetween the furnace wall and the workpiece, nitrogen ions are greatlyaccelerated and hit the workpiece surface at a high speed, with thejoint parts being heated by the energy released by the nigrogen ionshitting the workpiece. In time, the formation of nitride progresses dueto diffusion from the surface into the interior of the joint parts 2, 3.

Nitriding takes place at a temperature of approximately 580° C. heldover a period of approximately 3 hours. As a result of diffusion, thenitrogen, because of its small atom radius, can easily penetrate intothe iron grid of the upper layer of the joint parts 2, 3. In theprocess, the joint parts 2, 3 experience an increase in hardness, whichincrease results from the formation of nitrides and the dissolution ofnitrogen in the mixed iron crystal. After nitriding, the outer layer ofthe joint parts 2, 3 comprises an outer surface layer with iron nitridesand an adjoining diffusion zone with nitrogen-enriched mixed crystalsand partially precipitated nitrides.

After nitriding, the inner faces of the outer joint part 2, i.e. thesurfaces of the running grooves 7 and of the guiding face 6, as well asthe outer faces of the inner joint part 3, i.e. the surfaces of therunning grooves 9 and of the outer face 8, are induction-hardened. Inthe course of induction hardening, the outer layer of the joint parts 2,3 is heated in a middle frequency coil by induced current flows and,after the austenitising temperature has been reached, it is quenched ina shower or bath, and the polymer emulsion can be used as a coolant. Thedepth of the heated surface layer and thus also the surface layerhardness is reduced with an increasing frequency due to the skin effectWith a middle frequency of 10 kilohertz, the effective depth of hardnessamounts to approximately 1 to 2.8 mm and with a middle frequency of 30kilohertz, the effect depth of hardness amounts to approximately 1 to1.3 mm. However, the effective depth of hardness has to be kept as lowas possible in order to prevent effusion of the connecting zone.

After induction hardening, the joint parts 2, 3 are tempered to reducehardness stresses. Tempering takes place in an electrically heatedtempering furnace at a temperature of 175° C. for a period ofapproximately 1 hour.

FIG. 3 shows the structure of the outer layer of the outer joint part 2and of the inner joint part 3 after these have been nitrided andinduction hardened. The thickness of the illustrated layers is not toscale.

It can be seen that the surface-layer 15 comprises a pore seam 16 and,thereunderneath, a connecting zone 17. The pore seam 16 has a thicknessof approximately 2 to 3 micrometers. Together with the connecting zone17 with a thickness of approximately 7 to 27 micrometers there isobtained a total thickness of approximately 10 to 30 micrometers for thesurface layer 15. The thickness of the surface layer 15, inter alia,depends on the nitriding process used. In the case of plasma nitriding,it is possible to achieve surface thicknesses of 10 to 20 micrometers,whereas in the case of salt bath nitriding, layer thicknesses of 20 to30 micrometers are produced.

The pore containing seam 16 consists of burnt iron nitrides andcomprises a porous structure. It is thus possible for lubricant to enterthe pores when the joint is in operation, as a result of whichlubrication between the balls and the running groves of the constantvelocity plunging joint is improved considerably. In this way, thetendency of the joint parts to fret and wear is reduced considerably.The connecting zone 17 consists of ε-iron nitrides which comprise highhardness and high fatigue strength values and a high wear resistance.The connecting zone 17 thus improves the running characteristecs of theconstant velocity plunging joint, as a result of which the service lifeis prolonged.

The diffusion zone 18 positioned underneath the connecting zone 17consists of a ferritic-pearlitic matrix with nitrogen-enriched mixedcrystals and of precipitated nitrides. The depth of the diffusion zone18 up to which the nitrogen is diffused into the basic structure amountsto 0.8 mm. Said diffusion zone 18 is softer than the surface layer 15positioned thereabove, so that without any further heat treatment therewould be a risk of the surface layer 15 being pushed into the diffusionzone 18. Underneath the diffusion zone 18 there is positioned the core19 of the joint parts which remains mains unaffected by the nitridingprocess. The structure of the core 19 consists of pearlite and ferrite(normalised), with the grain size amounting to 5 to 8 according to ASTMStandard E 112 (American Society for Testing and Materials).

As a result of the induction hardening process following the nitridingoperation, the hardness of the joint part is increased further. Thethickness of the induction hardened layer 20 amounts to approximately1.8 to 2 mm. As can be seen, the induction hardened layer 20 issuperimposed on the diffusion zone 18 and extends beyond same. Thehardening resulting from induction hardening, during the quenchingoperation, leads to the formation of a martensite-nitrogen structure inthe diffusion zone 18 which comprises high hardness and strength values.The induction-hardened diffusion zone 18 is thus hard enough to supportthe relatively thin surface layer 15 which is in contact with the ballsand the cage respectively. Wear caused by the formation of pitting onthe running surfaces is reduced, as a result of which the service lifeof the constant velocity plunging joint is prolonged.

Components Made of Steel and Process of Heat Treating Components Made ofSteel

List of Reference Numbers

1 constant velocity joint

2 outer joint part

3 inner joint part

4 balls

5 cage

6 guiding face

7 running groove

8 outer face

9 running groove

10 window

11 bore

12 aperture

13

14

15 surface layer

16 pore containing seam

17 connecting zone

18 diffusion zone

19 core

20 induction hardened layer

1. A wear-resistant steel component comprising: a surface layer (15)comprising a pore containing seam (16) of burnt iron nitrides and,positioned thereunderneath, a non-metallic connecting zone (17) of ironnitrides; an adjoining diffusion zone (18) of nitrogen-enriched mixedcrystals and precipitated nitrides in a matrix of a martensite-nitrogenstructure; an induction hardened layer (20) at least in partsuperimposed on the diffusion zone (18); and a core (19) ofheat-treatable steel positioned thereunderneath.
 2. A componentaccording to claim 1, wherein the surface layer (15) formed of the porecontaining seam (16) and the connecting zone (17) extends over an entirecontact surface.
 3. A component according to claim 2, wherein at leastpart of the contact surface is induction-hardened.
 4. A componentaccording to claim 1, wherein the surface layer (15) formed of the porecontaining seam (16) and the connecting zone (17) has a thickness ofapproximately 10 to 30 μm.
 5. A component according to claim 4, whereinthe pore containing seam (16) has a thickness of approximately 2 to 3μm.
 6. A component according to claim 1, wherein the diffusion zone (18)has a thickness of approximately 0.8 μm.
 7. A component according toclaim 1, wherein the induction-hardened layer (20) has a thickness ofapproximately 1.8 to 2 μm.
 8. A component according to claim 1, whereinthe core (19) is normalised.
 9. A component according to of claim 1,wherein the steel is an unalloyed heat-treatable steel.
 10. A componentaccording to claim 9, wherein the steel consists of Cf
 53. 11. Acomponent according to claim 1, wherein the steel is alloy steel.
 12. Aprocess of heat-treating steel components comprising the followingsteps: nitriding a region of the component; thereafter, surface layerinduction hardening the region; and thereafter, tempering the region.13. A process according to claim 12, wherein the step of the nitridingincludes at least one of plasma nitriding, salt bath nitriding, or gasnitriding.
 14. (canceled)
 15. (canceled)
 16. A process according toclaim 12, wherein the step of nitriding is carried out at a temperatureof approximately 580° C.
 17. A process according to claim 12, whereinthe step of surface layer induction hardening takes place at a middlefrequency ranging between 10 and 30 kHz.
 18. A process according toclaim 12, wherein the step of tempering takes place at a temperature ofapproximately 175° C.
 19. A process according to claim 12, wherein thesteel components comprise unalloyed steel.
 20. A process according toclaim 12, wherein the steel components comprise alloy steel.
 21. Aprocess according to claim 12 comprising the step of normalizing thecomponents prior to them being nitrided.
 22. A process according toclaim 12 comprising heat-treating the components prior to them beingnitrided.